1. Field of the Invention
The present invention relates to a method for continuous tension stretching of thin strips, particularly metal strips of steel, aluminum or other metals having a thickness of between 0.05 mm and 0.5 mm. The method includes passing the strip through a set of brake rollers and a set of pull rollers and subjecting the strip to a stretch pull or stretch-forming operation between the two sets of rollers during stretching of the strip in the plastic range. The stretch pull force corresponds to or slightly exceeds the yield point of the strip material.
The present invention also relates to an arrangement for carrying out the above-described method.
2. Description of the Related Art
Thin metal strip is leveled by means of tension stretching arrangements with a set of brake rollers and a set of pull rollers. Both sets of rollers have two or more rollers to which staggered torques and, thus, stretch-forming forces are applied, so that finally the stretch pull or stretch-forming force required for the desired stretching is achieved between the set of brake rollers and the set of pull rollers.
During the stretching procedure, the plastic elongation of the strip is obtained by reducing the strip thickness and strip width. While the reduction of strip thickness is not impaired during the stretching procedure, the reduction of strip width is impaired in transverse direction because of the friction between the strip and the rollers, so that transverse tensions are built up from the strip edge toward the strip center with the result that greater plastic deformations occur in the middle portion of the strip than in the edge portions of the strip. In fact, the reduction of the strip width in transverse direction is particularly impaired because of attempts to build up high stretch-forming forces within as few rollers as possible and, consequently, rollers are used whose outer roller surfaces are provided with a friction lining of rubber, plastics material or the like. The last roller of the set of brake rollers and the first roller of the set of pull rollers have to transmit the maximum stretch-forming force to the strip in accordance with a staggering of the coefficient of friction e.sup..mu..alpha.. The stretch-forming forces to be generated are always adjusted in such a way that the respective yield point of the strip is reached between the last brake roller and the first pull roller and a strip elongation occurs which is freely selectable. The resulting longitudinal tensions cause the above-mentioned transverse tensions which depend on the strip material. The ratio of .epsilon..sub.transverse to .epsilon..sub.longitudinal, the so-called Poisson coefficient .mu., is in the range of between 0.25 to 0.3 in the case of metals.
If a longitudinal tension is applied to a strip, the strip is constricted in the elastic range as well as in the plastic range. The transverse tensions are built up increasingly from the edge to the center of the strip because the strip is prevented from transverse sliding on the rollers. In connection with the longitudinal tensions, this results in greater longitudinal deformations in the center of the strip. This is true for the elastic deformation as well as for the plastic deformation, so that more-or-less pronounced troughs are formed in the strip center which are not acceptable, particularly in strips to be used for lithographic purposes. In addition, during stretching in the plastic range, residual tensions occur in longitudinal direction which are uniformly distributed over the width of the strip.
If such a strip is cut into longitudinal strip portions to be used for different printing sizes, the edges in the center are slightly longer than the edges at the outside. This is also an undesirable effect. This effect is compounded by the fact that the strip can be pressed into the elastic coating of the roller surfaces, so that the strip edges at the edge portions of the strip are forced with greater force into the roller surface than are the middle strip portions.
It is, therefore, the object of the present invention to provide a method and an arrangement for continuous tension stretching of thin strip, particularly metal strip, of the above-described type in which the transverse changes of the strip occurring during plastic stretching are reduced to a minimum and, thus, the formation of center troughs and residual tensions non-uniformly distributed over the width of the strip are essentially eliminated.